Analytical devices suitable for home testing of analytes are now widely commercially available. An immunoassay device suitable for this purpose for the measurement of the pregnancy hormone human chorionic gonadotropin (hCG) is sold by Unipath under the brand-name CLEARBLUE® and is disclosed in EP291194.
EP291194 discloses an immunoassay device comprising a porous carrier containing: a particulate labelled specific binding reagent for an analyte, which reagent is freely mobile when in the moist state; and an unlabelled specific binding reagent for the same analyte, which reagent is immobilised in a detection zone or test zone downstream from the unlabelled specific binding reagent. Liquid sample suspected of containing analyte is applied to the porous carrier whereupon it interacts with the particulate labelled binding reagent to form an analyte-binding partner complex. The particulate label is coloured and is typically gold or a dyed polymer, for example latex or polyurethane. The complex thereafter migrates into a detection zone whereupon it forms a further complex with the immobilised unlabelled specific binding reagent enabling the extent of analyte present to be detected or observed. Due to the nature of the binding reactions taking place it is necessary to wait for a particular period of time to elapse after the test has commenced in order to read the result. This is particularly important for a visual, semi-quantitative type of test whereby the detection zone or read line develops over time.
Various methods of timing the result have been proposed for commercial devices, including instructions to the user wait for a particular length of time before reading the assay result. Other methods include a signal that is generated after a particular period of time has elapsed, as disclosed in our copending application no. PCT/EP03/00274 which signal informs the user that the assay result should now be read.
As a control and to ensure the correct functioning of the device, a control zone is generally provided downstream from the measurement zone. A third binding reagent, which is able to bind with the first labelled reagent, is immobilised at this control zone such that in the absence of analyte, the user will be able to check if the test has been carried out correctly.
EP653625 discloses a lateral flow assay test-strip for use in combination with an assay reader whereby the extent of binding of particulate label is determined optically. It is also known from U.S. Pat. No. 5,580,794 to provide an integrated assay device and lateral flow assay test-strip wherein the result is determined optically using reflectance measurements.
U.S. Pat. No. 5,837,546 discloses a method of automatically starting an immunoassay device whereby a lateral flow carrier is provided with additional electrodes which sense the presence of fluid on the test-strip and a signal is generated which switches on the sensing electronics.
Due to the nature of a lateral flow type test which requires the release of a labelled particulate binding reagent, flow of liquid along a carrier (typically porous) and capture of the analyte complex in the detection zone, it is desirable to optimise the properties of the porous carrier.
The pore size of the carrier is an important consideration and is preferably chosen to be between 1–12 μm. The carrier is conveniently nitrocellulose, the pore size of which may vary in part due to the manufacturing process. The assay device may additionally have a wick in fluid communication with the carrier which serves to collect the liquid sample and the carrier typically comprises two pieces of different materials. Nitrocellulose is typically used as the carrier material for the assay strip and has considerable advantages over conventional strip materials, such as paper, because of its natural ability to bind proteins without requiring prior sensitisation. In order to optimise the assay, the nitrocellulose is typically subjected prior to use to a number of treatments which include the use of blocking agents such polyvinylalcohol and the use of soluble glazes such as sugar to enhance release of the labelled reagent.
The present inventors have observed that the flow rate of fluid along the porous carrier may vary from test to test. In some cases the carrier has a tendency to flood, i.e. the fluid front moves along the carrier at a faster rate than normal. Conversely, in some cases, it has been noted that the fluid front moves along the carrier at a much slower rate than normal, namely the carrier is blocked to some extent. It has been found that these different types of fluid flow-rate behaviour can give rise to inaccurate results.
Due to the inconsistent nature of the materials used for both the wick and the porous membrane, the optimum point in time (after application of the liquid sample) for reading the result can be variable.
In the interests of providing devices which are inherently more accurate and reliable, it would be desirable to provide alternative or additional control features which would be able to determine the extent and/or rate at which the liquid sample moved along the porous carrier and to reject those readings where the flow rate was determined to fall outside of predetermined limits.
It would also be desirable to provide a method wherein the optimum time for reading the result could be reliably and reproducibly determined.